1. Field of the Invention
The present invention relates to a method of producing optical disks each by bonding two disk substrates to each other through an adhesive double coated sheet and apparatus used therefore.
2. Description of the Prior Art
An optical disk is formed by bonding two disk substrates having a recording layer in at least one surface to each other. The bonding of the disk substrates as described above is made by sandwiching an adhesive agent between the two disk substrates and pressing the two disk substrates against each other, as disclosed in, for example, Japanese Patent Publication No. Sho. 61-80534.
According to the bonding method using such an adhesive agent, there is, however, required a troublesome operation of applying the adhesive agent onto the disk substrates. Furthermore, it is difficult to apply the adhesive agent evenly, so that the degree of parallelism in the stuck disk substrates may become poor or air may be mixed in the adhesive agent so that air bubbles may be generated in a joint portion. Accordingly, there was also a problem in terms of quality. Further, when the disk substrates are pressurized, the adhesive agent is forced out from the peripheral portion of the disk substrates so that burrs are produced. Accordingly, the method was not preferred also from the point of view of producing efficiency, operating efficiency, etc. because of the necessity of deburring, or the like.
Therefore, an optical disk producing apparatus described in Japanese Patent Publication No. Hei. 3-230338 has been proposed as an apparatus for bonding two disk substrates without use of any adhesive agent. As shown in FIG. 16, this apparatus comprises a disk substrate supply device 41, an adhesive double coated sheet supply device 42, a bonding jig 43, a jig opening/closing device 44, a conveyor 47 for conveying and inverting the disk substrates 46, a vacuum presser 48, a jig moving device 52, a release film gripping mechanism 49, and a finished article storage place 50. The two disk substrates 46 are bonded through an adhesive double coated sheet 51 by means of vacuum pressing. That is, the two disk substrates 46 are bonded to each other by the steps of: first, bonding the first disk substrate 46 to the adhesive double coated sheet 51 by means of vacuum pressing; then separating a release film 53 by the release film gripping mechanism 49 and inverting the first disk substrate; and then bonding the second disk substrate 46, by means of vacuum pressing, to the first disk substrate 46 with the adhesive double coated sheet 51 stuck thereto.
In the aforementioned apparatus, vacuum pressing, however, must be required twice for bonding two disk substrates 46 to each other to obtain one optical disk. Furthermore, because a large time is required for evacuation whenever the vacuum pressing is made, producing efficiency is very poor. As a result, there arises a problem that the cost increases. Further, because this apparatus requires a vacuum pump used for vacuum pressing and the frequency of use of the vacuum pump is high, maintenance, or the like, for the vacuum pump is required so that the cost increases more intensively. There is a further problem that the optical disk bonded by means of vacuum pressing is large in distortion and warp and low in precision.